This invention relates to the application of plastic coatings to workpieces, and is more particularly concerned with a method and apparatus whereby a three-dimensional workpiece (as distinguished from one that is sheet-like) can be given a coating over its entire surface that is regular and unbroken, with no marks that disclose where the workpiece was supported by aa holder or a tool during the coating process.
The general problem that is solved by the present invention, and some of the more general objects that are achieved by it, are discussed in U.S. Pat. No. 3,573,953, to J. M. Laulan. The Laulan patent contemplates coating a workpiece by immersing it, while heated, in a so-called fluidized bed. In a fluidized bed, heat fusible plastic material in finely divided solid form is held in more or less agitated suspension by air blown upwardly through the bottom wall of a vessel in which the fluidized bed is contained. The airborne plastic particles are brought into contact with the surface of the heated workpiece, upon which they melt and fuse into a coating. The Laulan patent brings out that an article to be coated must be suspended in a fluidized bed for a substantial period of time in order to obtain a complete coating, and that when tongs, hooks or other tools are used for supporting articles in a fluidized bed, such tools leave marks or uncoated spots in the coating.
To avoid such defects, the Laulan patent proposed to allow the workpiece to fall, unsupported, into and partway through the fluidized bed. Laulan taught that different amounts of air should be blown up from different areas of the bottom wall of the fluidized bed vessel, so that the bed therein would have different densities in different parts of the vessel, being less dense and more fluidized over those areas of the bottom wall through which more air was blown. The heated workpiece was to be dropped into a part of the vessel in which the bed was most dense, and its lowermost surface was thus coated as it fell. It landed on the upper end of an inclined grid in the vessel, spaced below the surface of the fluidized bed, and it moved downwardly along the grid by gravity, receiving the rest of its coating as it did so. The workpiece was removed from the fluidized bed by raising the grid.
Laulan did not completely solve the problem posed by the need for supporting the workpiece. At most the process promised a reasonable certainty that the coating would cover all portions of the workpiece, but it afforded no certainty that the finished coating would be unblemished. The workpiece could be completely coated -- initially, at least -- because all portions of its surface were exposed to impact by plastic particles during the course of its fall through the fluidized bed. But the time during which the workpiece was thus unsupported obviously could not be long enought for accomplishing both coating of the workpiece and substantial hardening of the newly applied coating. At the end of its fall the workpiece encountered the grid, which thereafter served as a holder for the workpiece and which could leave its mark in the soft coating, like any other holder. Furthermore, the collision of the workpiece with the grid presented the possibility of a scraping by which some of the soft coating might be removed, leaving an exposed area on the workpiece.
Even if a complete coverage of the workpiece could be assured with the procedure disclosed by Laulan, and assuming that blemishes in the coating could be somehow avoided, or could be ignored, the thickness of the finished coating could not be controlledly varied to any substantial extent, inasmuch as it depended upon the duration of the very brief period of fall through the fluidized bed.